Hub unit bearings comprising a rolling bearing unit are used for supporting the wheels of an automobile so as to be able to rotate freely with respect to the suspension. In recent years, rotational speed detectors for detecting the rotational speed of the wheels have been installed into this kind of hub unit bearing, and control of anti-lock brake systems (ABS) or traction control systems (TCS) is widely performed.
As an example of this kind of hub bearing unit with rotational speed detector, a structure such as illustrated in FIG. 24 is disclosed in JP2005-090638. The hub unit bearing with rotational speed detector of this first example of conventional construction comprises a hub unit bearing 1 and a rotational speed detector 5, and the hub unit bearing 1 comprises an outer ring member 2, a hub 3, which is an inner ring member, and a plurality of balls 4, which are rolling elements.
The outer ring member 2 has a plurality of outer raceways 6 formed around the inner circumferential surface thereof, and a stationary side flange 7 around the outer circumferential surface. The outer ring member 2 corresponds to a stationary ring that is supported by the knuckle (not illustrated in the figure) of the suspension and does not rotate during operation.
The hub 3 is a combination of a main hub 8 and inner ring 9, and has a plurality of inner raceways 10 formed around the outer circumferential surface, and is supported on the inner diameter side of the outer ring member 2 such that it is concentric with the outer ring member 2. A rotating side flange 11 for supporting the wheel is formed on a portion of the outside end in the axial direction of the main hub 8, that protrudes further outward in the axial direction than the opening on the outside end in the axial direction of the outer ring member 2. Moreover, spline holes 13 for making a spline fit with the drive shaft (not illustrated in the figure) that is fastened to the surface on the outside end in the axial direction of an outer ring 12 of a constant velocity joint is provided in the center section of the main hub 8. A plurality of balls 4 is located between each of the outer raceways 6 and the inner raceways 10 so as to be able to roll freely. The outside in the axial direction is defined as the side toward the outside in the width direction of the vehicle body when installed in the suspension, and the inside in the axial direction is defined as the side that is near the center section in the width direction of the vehicle body.
A seal ring 14 is provided between the opening section on the outside end in the axial direction of the outer ring member 2 and the outer circumferential surface of the middle section in the axial direction of the main hub 8. This seal ring 14 covers the opening on the outside end in the axial direction of the rolling element installation space 15 where the balls 14 are located, and prevents grease that is inside this space 15 from leaking to the outside, and prevents foreign matter on outside from entering into the space 15. On the other hand, a combined seal ring 16 is provided between the portion around the outer circumferential surface on the inside end section in the axial direction of the inner ring 9 that is separated toward the inside in the axial direction away from the inside inner raceway 10 that is formed around the inner ring 9 and the inner circumferential surface on the inside end section in the axial direction of the outer ring member 2, and covers the opening on the inside end in the axial direction of the space 15.
A cover 17 is fastened around the outside of the inside end section in the axial direction of the outer ring member 2. This cover 17 is formed into a circular ring shape by pressing metal plate, and a seal member 18 made using an elastic material is attached around the inner edge of the inside end in the axial direction. The edges of the tip ends of a plurality of seal lips that are formed on the seal member 18 come in sliding contact all the way around the outer circumferential surface and stepped surface on the outside end section in the axial direction of the outer ring 12 for a constant velocity joint.
On the other hand, the rotational speed detector 5 comprises an encoder 19 and sensor 20. The encoder 19 is such that the characteristics of the inside surface in the axial direction, which is the detected surface, alternate at uniform intervals in the circumferential direction, is supported and fastened such that it is concentric with the hub 3 and rotates together with the hub 3. In the example in the figures, an encoder 19, which is made using permanent magnets with the S-poles and N-poles alternating around the inside surface in the axial direction, is attached and fastened to the inside surface in the axial direction of the slinger 21 of the combined seal ring 16. Moreover, the sensor 20 has a magnetic detecting element such as a Hall element or magnetic resistance element that is provided in a detecting section, and is supported by and fastened to the cover 17. In this state, the detecting section of the sensor 20 faces the inside surface in the axial direction of the encoder 19. Furthermore, of a sensing space 22 that is located in the detecting section of the encoder 19 and the sensor 20 is such that the opening on the inside end in the axial direction is covered by the seal member 18, and the opening on the outside end in the axial direction is covered by the combined seal ring 16.
With the first example of conventional construction of a hub unit bearing 1, the wheel that is fastened to the hub 3 can supported such that it rotates freely with respect to the suspension the supports the outer ring member 2. Moreover, as the encoder 19 rotates together with the hub 3 as the wheel rotates, the N-poles and S-poles on the detected surface of the encoder 19 alternate in passing the detecting section of the sensor 20. As a result, the direction of the magnetic flux that flows in the magnetic detection element of the sensor 20 changes, and the characteristic of this magnetic detection element alternately changes. The frequency at which the characteristics of the magnetic detection element changes in this way is proportional to the rotational speed of the hub 3, so by sending the detection signal from the sensor 20 to a controller (not illustrated in the figure), it is possible to perform suitable ABS or TCS control. Furthermore, in the case of the first example of conventional construction, the sensing space 22 can be closed off from the outside space by the seal member 18 that is attached to the cover 17. Therefore, it is possible to prevent foreign matter such as sand or small stones from entering in and biting in between the inside surface in the axial direction of the encoder 19 and the detecting section of the sensor 20, and thus it is possible to protect the encoder 19 and sensor 20 from the danger of damage. As a result, the reliability of the rotational speed detection can be maintained, and suitable ABS or TCS control is possible.
However, even in the case of the first example of conventional construction, there is a possibility of moisture or minute particles entering into the sensing space 22 through a minute space between the seal member 18 and the outer ring 12 of the constant velocity joint, or through a minute space between the cover 17 and the outer ring member 2. Therefore, as the bearing is used over a long period of time, foreign matter may accumulate inside the sensing space 22, which causes a drop in reliability of the rotational speed detection.
For such a problem, as disclosed in JP2008-175382(A), JP2005-140320(A), JP2005-331429(A) and JP2005-009525(A), the installation of a water drainage hole is performed. FIG. 25 shows a second example of conventional construction of hub unit bearing 1a which is disclosed in JP2008-175382(A). In the case of this second example of conventional construction, a water drainage hole 23 is formed in the portion of a cover 17a that is fastened to the inside end section in the axial direction of the outer ring member 2 that is located on the bottom end during operation. More specifically, the cover 17a comprises a large-diameter cylindrical section 24 for fastening around the inside end section in the axial direction of the outer ring member 2, a circular ring shaped circular disk section 25 that is bent at a right angle toward the inside in the radial direction from the inside end section in the axial direction of the large-diameter cylindrical section 24, and a small-diameter cylindrical section 26 that is bent at a right angle toward the inside in the axial direction from the inside end section in the radial direction of the circular disk section 25. The water drainage hole 23 is formed in the inner half section in the axial direction of the large-diameter cylindrical section 24 so as to pass through the large-diameter cylindrical section 24, connecting the inside and outside of the cover 17.
In this second example of conventional construction, foreign matter such as moisture or minute particles that has entered into the sensing space 22 can be discharged to the outside space through the water drainage hole 23. Therefore, it is possible to prevent foreign matter from accumulating inside the sensing space 22, and thus it is possible to maintain reliability of the rotational speed detection. It is omitted in the figures, however, in the case of the construction of the invention disclosed in JP2005-140320(A), JP2005-331429(A) and JP2005-009525(A) as well, a water drainage hole is formed in the portion of the cover that is located at the bottom of the cover during operation. Therefore, as in the case of the second example of conventional construction, it is possible to discharge foreign matter that entered into the sensing space to the outside space.
Incidentally, in any of the construction disclosed in JP2008-175382(A), JP2005-140320(A), JP2005-331429(A) and JP2005-009525(A), including the second example of conventional construction, the only intention for the water drainage hole is to discharge foreign matter to the outside space, and preventing foreign matter from entering from that outside space through this water drainage hole is not particularly considered. In other words, when foreign matter such as water from a car wash, or dirty water that is splashed on the vehicle during operation, much of that foreign matter is comes near the cover 17 from underneath. As can be clearly seen in FIG. 25, a water drainage hole 23 that is formed in the cover 17a is a simple hole that is formed in the bottom section of the large-diameter cylindrical section 24, and as seen from the bottom of the vehicle, the entire opening section of the water drainage hole 23 is exposed. Consequently, it is easy for foreign matter such as dirty water to enter inside the cover 17a through this water drainage hole 23. Therefore, there is a possibility that the reliability of rotational speed detection will drop due to foreign matter adhering to the inside surface in the axial direction of the encoder and to the detecting section of the sensor 20. Moreover, there is a possibility that the strength of the portion of the cover 17 which is fitted around the outer ring member 2 will decrease with location where the water drainage hole 23 is formed.